Not really, resistors only change the voltage when there is enough current, we are talking about the thresholds where current just starts to flow (since the retina's response is very logarithmic we can easily detect a dim red light in the presence of a bright blue one. The red LEDs might be carrying 0.1mA while the blue 20mA, any resistor that allows the 20mA through when needed is going to drop only millivolts when 0.1mA is flowing. Diodes on the other hand will drop 0.5V or so for small current and 0.7V for large.

[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]